Allan Hills 84001 (commonly abbreviated ALH84001[1]) is a meteorite that was found in Allan Hills, Antarctica on December 27, 1984 by a team of U.S. meteorite hunters from the ANSMET project. Like other members of the group of SNCs (shergottite, nakhlite, chassignite), ALH84001 is thought to be from Mars. However, it does not fit into any of the previously discovered SNC groups. On discovery, its mass was 1.93 kilograms (4.3 lb). It made its way into headlines worldwide in 1996 when scientists announced that it might contain evidence for microscopic fossils of Martian bacteria based on carbonate globules observed.

This rock is considered to be one of the oldest pieces of the Solar System, proposed to have crystallized from molten rock 4.091 billion years ago.[2] Based on chemical analyses, it is thought to have originated on Mars[3][4] from a period when liquid water existed on the now barren planet's surface.[5][6]

The theory holds that ALH84001 was blasted off from the surface of Mars by a meteorite impact about 17 million years ago[8] and fell on Earth roughly 13,000 years ago.[9] These dates were established by a variety of radiometric dating techniques, including samarium-neodymium (Sm-Nd), rubidium-strontium (Rb-Sr), potassium-argon (K-Ar), and carbon-14.[10][11] Other meteorites that have potential biological markings have generated less interest because they do not originate from a "wet" Mars. ALH84001 is the only meteorite collected from such a time period.[12]

In October 2011 it was reported that isotopic analysis indicated that the carbonates in ALH84001 were precipitated at a temperature of 18 °C with water and carbon dioxide from the Martian atmosphere. The carbonate carbon and oxygen isotope ratios imply deposition of the carbonates from a gradually evaporating subsurface water body, probably a shallow aquifer meters or tens of meters below the surface.[6]

Under the scanning electron microscope structures were revealed that some scientists interpreted as fossils of bacteria-like lifeforms. The structures found on ALH84001 are 20–100 nanometres in diameter, similar in size to theoretical nanobacteria, but smaller than any known cellular life at the time of their discovery. If the structures are in fact fossilized lifeforms, as proposed by the so-called biogenic hypothesis of their formation, they would be the first solid evidence of the existence of extraterrestrial life, aside from the chance of their origin being terrestrial contamination.[14]

The announcement of possible extraterrestrial life caused considerable controversy. When the discovery was announced many immediately conjectured that the fossils were the first true evidence of extraterrestrial life—making headlines around the world, and even prompting the President of the United StatesBill Clinton to make a formal televised announcement to mark the event.[15]

David S. McKay at NASA argues that likely microbial terrestrial contamination found in other Martian meteorites do not resemble the microscopic shapes in the ALH84001. In particular, the shapes within the ALH84001 look intergrown or embedded in the indigenous material, while likely contamination do not.[16] While it has not yet conclusively been shown how the features in the meteorite were formed, similar features have been recreated in the lab without biological inputs by a team led by D.C. Golden.[17] David McKay says these results were obtained using unrealistically pure raw materials as a starting point,[12] and "will not explain many of the features described by us in ALH84001." According to McKay, a plausible inorganic model "must explain simultaneously all of the properties that we and others have suggested as possible biogenic properties of this meteorite."[17]

In November 2009, a team of scientists at Johnson Space Center, including McKay, argued that since their original paper was published, the biogenic hypothesis has been "further strengthened by the presence of abundant fossil-like structures in other Martian meteorites."[16] However, the scientific consensus is that "morphology alone cannot be used unambiguously as a tool for primitive life detection."[18][19][20] Interpretation of morphology is notoriously subjective, and its use alone has led to numerous errors of interpretation.[18]

Of the various tests used, three have been shown to be particularly useful:[18] (1) the mm-scale morphology of the objects in question; (2) the carbon isotopic composition of organic matter associated with and/or comprising the fossil-like structures; and (3) the chemical (molecular) makeup of the fossil-like objects. In principle, therefore, the question of biogenicity can be easily answered. But in practice, the answer has proven elusive, primarily because of a lack of analytical techniques having sufficient power to provide the high resolution three-dimensional morphological information needed to definitively address the question, and an absence of means by which to directly link, in individual microscopic specimens, morphological information to elemental-isotopic and structural-molecular compositions.[18]